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“We still talk about the seven layers model, because it’s a convenient model for discussion, but that has absolutely zero to do with any real-life software engineering. In other words, it’s a way to talk about things, not to implement them. And that’s important. Specs are a basis for talking about things. But they are not a basis for implementing software.”– Linus Torvalds, project coordinator for the Linux kernel, in an e-mail dated September 29, 2005 (http://lkml.org/lkml/2005/9/29/233).

The Open systems Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

Layer 1: Physical
The first layer of the OSI model is the Physical layer, which specifies the electrical and mechanical requirements for transmitting data bits across the transmission medium (cable or airwaves). It involves sending and receiving the data stream on the carrier, whether that carrier uses electrical (cable), light (fiber optic), radio, infrared, or laser (wireless) signals. The Physical layer specifications include:

• Voltage changes
• The timing of voltage changes
• Data rates
• Maximum transmission distances
• The physical connectors to the transmission medium (plug)
• The topology or physical layout of the network

Layer 2: Data Link
The Data Link layer is responsible for maintaining the data link between two computers, typically called hosts or nodes. It also defines and manages the ordering of bits to and from packets. Frames contain data arranged in an organized manner, which provides an orderly and consistent method of sending data bits across the medium. Without such control, the data would be sent in random sizes or configurations and the data on one end could not be decoded at the other end. The Data Link layer manages the physical addressing and synchronization of the data packets. It is also responsible for flow control and error notification on the Physical layer. Flow control is the process of managing the timing of sending and receiving data so that it doesn’t exceed the capacity of the physical connection or host. Since the Physical layer is only responsible for physically moving the data onto and off of the network medium, the Data Link layer also receives and manages error messaging related to the physical delivery of packets.

Layer 3: Network
The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible.

Layer 4: Transport
The Transport layer is responsible for transporting the data from one node to another. It provides transparent data transfer between nodes, and manages the end-to-end flow control, error detection, and error recovery.
The Transport layer protocols initiate contact between specific ports on different host computers, and set up a virtual circuit. Transmission Control Protocol (TCP) is one such layer 4 protocol. As an example, TCP verifies that the application sending the data is authorized to access the network and that both ends are ready to initiate the data transfer. When this synchronization is complete, the data is sent. As the data is being transmitted, the TCP protocol on each host monitors the data flow and watches for transport errors. If transport errors are detected, TCP provides error recovery.

Layer 5: Session
The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.

Layer 6: Presentation
Data translation is the primary activity of the Presentation layer. When data is sent from a sender to a receiver, it is translated at the Presentation layer (i.e., the sender’s application passes data down to the Presentation layer, where it is changed into a common format). When the data is received on the other end, the Presentation layer changes it from the common format back into a format that is useable by the application. Protocol translation (i.e., the conversion of data from one protocol to another so that it can be exchanged between computers using different platforms or OSes) takes place here.

Layer 7: Application
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

The OSI model is generic, yet provides the appropriate guidelines to be used to explain the majority of network protocols. Various protocol suites are often mapped against the OSI model for this purpose. A solid understanding of the OSI model aids in network analysis, comparison, and troubleshooting. However, it is important to remember that not all protocols map well to the OSI model (e.g., TCP/IP was designed to map to the U.S. Department of Defense (DoD) model). In the 1970s, the DoD developed its four-layer model. The core Internet protocols adhere to this model.

The DoD model is a condensed version of the OSI model. Its four layers are:

• Application/Process Layer This layer defines protocols that implement user-level applications (e.g., e-mail delivery, remote login, and file transfer.
• Host-to-host Layer This layer manages the connection, data flow management, and retransmission of lost data.
• Internet Layer This layer delivers data from the source host to the destination host across a set of physical networks that connect the two machines.
• Network Access Layer This layer manages the delivery of data over a particular hardware media.

Resources:

• OSI model - Wikipedia, the free encyclopedia, http://en.wikipedia.org/wiki/OSI_model, July 21 2009
• Nmap in the Enterprise: Your Guide to Network Scanning, Oreabugh and Pinkard, 2008